The present invention relates generally to motor drive systems and, more particularly, to a system and method for suppressing harmonics injected onto AC power lines when operating a motor drive unit.
Often, power delivered from a power source or supply is not properly conditioned for consumption. For example, power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids that are designed to be extremely efficient at delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as fixed frequency three-phase alternating current (AC) power. As such, the power must typically be converted or “conditioned” prior to consumption.
For example, motors and their associated loads are one type of common inductive load employed at many consuming facilities that require power conditioning. When a motor is the consuming point, power “conditioning” systems are utilized to convert the fixed frequency AC power delivered over utility grids to a form suitable for driving the motor. To this end, power conditioning for motor systems typically include AC-to-DC (direct current) rectifiers that convert the utility AC power to DC power applied to positive and negative DC buses (i.e. across a DC link). The power distributed across the DC buses is then converted, for example, by use of an inverter, to AC power designed to drive the motor.
Specifically, referring to FIG. 1, the present invention will be described with respect to a motor system 10. The motor system 10 generally includes a power supply 12, a motor drive unit 14, and a motor 16. The power supply 12 provides power to the motor drive unit 14 that, in turn, converts the power to a more usable form for the motor 16 that drives an associated load 18.
The motor drive unit 14 includes a variety of components, such as a rectifier 20, an inverter 22, and a controller 24. During operation, the power supply 12 provides three-phase AC power, for example, as received from a utility grid over transmission power lines 26. However, it is also contemplated that the power supply 12 may deliver single-phase power. The rectifier 20 is designed to receive the AC power from the power supply 12 and convert the AC power to DC power that is delivered to positive and negative DC buses 28, 30 of a DC link 32. Specifically, the rectifier 20 includes a plurality of switches that are controlled by the controller 24 using pulse-width modulation (PWM) to rectify the AC power received from the AC power lines 26.
The DC power is delivered from the rectifier 20 over the positive and negative DC buses 28, 30 to the inverter 22. The inverter 22 includes a plurality of switching devices (e.g., IGBTs or other semiconductor switches) that are positioned between the positive and negative buses 28, 30 and controlled by the controller 24 to open and close specific combinations of the switches to sequentially generate pulses on each of the supply lines 34 to drive the motor 16 and, in turn, the load 18 through a drive shaft 36.
While PWM control of the rectifier 20 is an efficient means for converting the AC power received from the power lines 26 to DC power, high-frequency harmonics can be injected onto the power lines 26 as a result. These injected high-frequency harmonics can adversely affect operation of the motor drive unit 14 and other systems connected to the power supply 12.
As a result, a filter 38 is often arranged between the power supply 12 and the motor drive unit 14. Typically, an inductor L1 is arranged to act as a filter to suppress high-frequency harmonics injected onto the power lines 26 by operation of the motor drive unit 14. However, in many cases, the inductor L1 is unable to sufficiently attenuate the high-frequency harmonics.
Accordingly, an additional inductor Lm and a capacitor C1 are sometimes included to construct an LCL filter. While the LCL filter is typically insufficient to suppress all harmonics from being injected onto the power lines 26, it provides increased attenuation over the simple inductor L1 filter, such that the harmonics injected are tolerated by the system.
However, as current power regulations, such as IEEE 519, become more and more stringent, the amount of high-frequency harmonics tolerated under the regulations decrease. Furthermore, since harmonics regulations vary between countries, the amount of harmonics tolerated under these regulations varies by country. Accordingly, motor drive units must be tailored to meet the requirements of each country or must be designed to meet the most stringent of harmonics regulations.
It would be desirable to have a system and method for suppressing or substantially eliminating harmonics injected onto AC power lines when operating a motor drive unit.